EP0534195A1 - Reactor and process for carrying out heterogeneous catalytic reactions - Google Patents

Abstract

The application relates to a reactor and a process for carrying out heterogenous catalytic gas-phase reactions. The process is characterised in that the reaction mixture flows through a loose catalyst bed (01) radially with respect to the longitudinal axis of the reactor and preferably in a centripetal direction. A further characteristic of the process according to the invention is the incorporation, in the catalyst bed (01), of a heat exchanger (02) having coiled tubes (03) which have a heat transfer medium flowing through them to remove the reaction heat liberated. <IMAGE>

Description

The present invention relates to a reactor and a method for carrying out heterogeneous gas phase reactions.

Examples of such heterogeneous catalytic gas phase reactions include the production of ethylene oxide from ethylene and oxygen, the production of acrylic acid from acrolein and oxygen, the production of methyl acrylic acid from methacrolein and oxygen, the production of styrene by dehydration of methylphenylcarbinol, the production of methanol from synthesis gas, the methanation of synthesis gas, the low-temperature conversion, the conversion of water with olefins to alcohols, the oxychlorination of olefins or aromatics, or the production of methylamines from methanol and ammonia. However, the examples cited in no way represent a complete list of all the reactions which can be carried out using the reactor and process according to the invention.

Reactor types for carrying out heterogeneous gas phase reactions are known in large numbers. A straight tube bundle reactor is used in the vast majority of all technically applied heterogeneous catalytic gas phase reactions. Such reactors have long been state of the art and are manufactured and sold commercially by numerous companies. The waste heat generated in straight tube bundle reactors is dissipated using a suitable cooling medium, such as pressurized water or high-boiling hydrocarbons, and is mostly used for steam production.

This process, in particular the straight tube bundle reactor used, has some disadvantages inherent in it. For example, the length of the catalyst bed and thus of the reactor that the gas mixture traverses must be relatively large for satisfactory conversion, as a rule a few meters. This leads to high costs for gas compression, since the pressure loss over the length of the reactor is usually considerable. Another disadvantage of this type of reactor is that the capacity of such tube bundle reactors is limited. The reactors consist of up to several thousand individual tubes, which are combined in one reactor body. An increase in the production output of such a reactor is not possible by increasing the diameter of the individual tubes, since in this case the average distance of the individual catalyst particles from the cooled or heated tube wall becomes greater. This makes the cooling worse, so that individual, overheated, catalyst particles can act as ignition sources for a "runaway" reaction. The high heat of reaction released in such a case generally causes irreversible damage to the catalyst. An increase in the production output of a tube bundle reactor is therefore only possible by increasing the number of tubes with a suitable diameter of the individual tubes. However, this procedure also leads to problems. Since, with the exception of salt bath-cooled reactors for reactions at temperatures above 300 ° C, the required pressure resistance of the reactor body must be based on the high pressure of the coolant and not on the lower pressure of the reaction gas mixture, these reactors are very thick-walled, heavy, complex and therefore also manufactured expensive and can only be transported with great difficulty. The sizes realized today already reach the limit of what makes economic sense. Another major disadvantage of this type of construction is that the straight tube bundle is very susceptible to thermal stresses which are caused by temperature differences between the reactor pressure vessel wall and the reaction tube wall. As a result, there is a risk that reaction tubes will kink or tear out due to improper operation or serious malfunctions, or that the reactor pressure vessel wall will tear open.

A fundamentally different type of reactor for carrying out heterogeneous catalytic gas phase reactions is based on the use of a loose catalyst bed with an embedded heat exchanger or, in the case of exothermic reactions, another suitable type of heat dissipation. Here, too, the state of the art is great and is documented in numerous documents. For example, US-A-2 744 813 claims a reactor with a loose catalyst bed, straight cooling tubes and axial gas flow, EP-B 1 82 609 (ICI) a similar reactor with a radial gas flow. However, reactors with straight cooling tubes in a loose catalyst bed require flat tube sheets and therefore suffer from the same problems as a conventional straight tube bundle reactor. Furthermore the filling and emptying of the catalyst is much more difficult. Another type of reactor with loose catalyst bed and embedded heat exchanger uses a heat exchanger made of layers of coiled tubes (DE 34 14 717 (Linde), DE 28 48 014 (Linde), DE 39 35 030 (Linde)). With coiled tubes there is no need for flat tube sheets and problems with thermal stresses are practically non-existent due to the elasticity of the tube coils. In this type of construction, the pipes are flowed across, due to the special geometry of the pipe arrangement, the gas mixing and thus the convective heat transport is greatly increased. In addition, the average distance between a catalyst grain and the next heat exchange surface is smaller than in comparable straight tube bundle reactors. Both effects lead to a significant reduction in the required heat exchange area compared to straight tube bundle reactors of the same catalyst volume to about half. The specific heat exchange surface can also be adapted to the progress of the reaction or the temperature of the coolant flowing in the tubes can be varied (EP 339 748 A2 (Shell)). A further heat transfer possibility consists in admixing reactants or inert gases along the catalyst bed (DE 28 48 014, DE 39 35 030) or, in the case of exothermic reactions, in passing a cold reactor inlet gas through the catalyst bed (US Pat. No. 1,835,827 (Du Pont)) .

A phenomenon which can be observed in practically every heterogeneous catalytic gas phase reaction is that the product of value of the reaction is at least partially decomposed again by contact with the catalyst used for its synthesis or under the reaction conditions prevailing in the catalyst bed, so that part of that formed in the front part of the catalyst bed Product in the rear part is destroyed again. As has now been found, contrary to the prevailing opinion, this residence time effect is more important for the yield than a moderate increase in temperature. In the event of exothermic decomposition of the product of value, for example the further oxidation of the product of selective oxidation with oxygen to carbon dioxide, the catalyst can be irreversibly damaged by going through the reaction with excessive heat development. In other reactions, the catalyst can be passivated by decomposing the product of value with coke separation. Another, disturbing Influence of the product of value present in the reaction gas mixture can be the inhibition of the desired reaction by the product of value. In each of the cases shown, it is advantageous to remove the product of value from the catalyst bed as quickly as possible.

An obvious procedure to alleviate the problem of decomposition of the product of value as it passes through the catalyst bed, as described above, is to keep the catalyst path traversed by the reaction gas mixture as short as possible. However, the conversion of starting materials to the product of value drops, which can usually be compensated for by increasing the temperature if this increase in temperature has no negative effects on the reaction. In addition, the pressure drop across the reactor and thus the compression costs can be reduced by shortening the catalyst bed, or, alternatively, the production output of the reactor can be increased by increasing the amount of reaction gas passed through the catalyst per unit of time. In processes that work according to the principle of the cycle gas mode with a continuous discharge of products, the pressure loss can be minimized by shortening the catalyst layer to such an extent that such an increase in the amount of reaction gas passed through the catalyst bed is possible that with constant production of valuable product the Degree of conversion per reactor run can be reduced. This usually has the additional advantage of improved selectivity, so that less starting material has to be used for a production which is kept constant than in the conventional mode of operation. However, with an increased gas throughput and a reduced degree of conversion, it must be ensured that the processing and cleaning system can cope with the resulting gas quantities. The optimization of reactors and processes in the parameter field outlined here is the usual task of a person skilled in the art in this area.

The object of the present invention is to find an improved process for carrying out heterogeneous catalytic gas-phase reactions and a reactor for carrying out the reactions.

Improved processes for performing such reactions, such as the production of ethylene oxide from ethylene and oxygen, the production of acrylic acid from acrolein and oxygen, the Production of methacrylic acid from methacrolein and oxygen, the production of styrene by dehydration of methylphenylcarbinol, the production of methanol from synthesis gas, the methanation of synthesis gas, the low-temperature conversion, the conversion of water with olefins to alcohols, the oxychlorination of olefins or aromatics or the production of methylamines from methanol and ammonia are of great economic importance.

This object is achieved by the characterizing features of claim 1.

Further features of the method according to the invention are the subject of subclaims 2-10.

Claims 11-20 show processes for the production of valuable products by means of the reactor and process according to the invention shown.

A shortening of the catalyst bed to be traversed by the reaction gas mixture can be achieved particularly advantageously by the fact that the reaction gas mixture does not flow through shortened catalyst-filled tubes of a conventional type of reactor, but instead a loose catalyst bed is used, through which the reaction gas mixture does not flow in the axial but in the radial direction becomes. The direction of flow can be centrifugal or centripetal, the centripetal direction being particularly advantageous since the flow velocity of the gas in this case increases continuously from the entry into the reactor to the exit from the reactor. The residence time of the reaction gas mixture in a certain layer thickness of the catalyst bed consequently decreases with the progress of the reaction. As a result, already formed product of value is removed from the catalyst bed more quickly and any subsequent reactions which reduce yield are removed. The case of a centrifugal flow of the reaction gas mixture can have advantages over a centripetal flow, however, in particular circumstances, particularly in the case of reactions which increase the volume considerably. In the case of such reactions, by appropriate dimensioning of the reactor or by other measures, for example the installation of displacement bodies on the outer edge of the reactor, a residence time optimization can also be carried out with centrifugal flow.

However, the very special advantage of the reactor and process according to the invention in its normally preferred embodiment is the accelerated removal of the product of value from the catalyst bed with a centripetal gas stream. The negative effects of the product of value being in the catalyst bed for too long, for example loss of yield due to its decomposition with possible damage to the catalyst, for example due to excess temperatures in the event of total oxidation of the product of value, selective oxidation with oxygen or coking of the catalyst, are counteracted with the inventive reactor and process as effectively as passivation of the catalyst by product of value formed. As already mentioned above, the effect of a too long residence time on the yield of valuable products is more important than the effect of an increased reaction temperature. This makes it possible to raise the reaction temperature by means of thermal measures with the progress of the reaction in the event of an unsatisfactory degree of conversion, thus completing the conversion.

Figur 1

zeigt einen Querschnitt durch den erfindungsgemäßen Reaktor,

Figur 2

zeigt einen Querschnitt durch den erfindungsgemäßen Reaktor mit zusätzlich regelbarem Heizkreisumlauf.

An embodiment of the invention with the essential inventive features is shown in the drawing and will be described in more detail below.

Figure 1

shows a cross section through the reactor according to the invention,

Figure 2

shows a cross section through the reactor of the invention with an additional controllable heating circuit.

It is absolutely necessary to remove the heat of reaction that arises. The use of a radial flow reactor is particularly advantageous here, in which a tube-bundle heat exchanger (02) with wound cooling tubes (03) is installed in the catalyst bed (01). A suitable heat transfer medium flows in these, for example pressurized water or kerosene. In one possible and advantageous design, the tube bundle consists of a plurality of concentric tube layers which start from the hemisphere bottom of the distributor (04) and are brought together again on the hemispherical bottom of the header (05). The number, spacing in the horizontal and vertical directions, and the size and direction of the pitch of the individual windings can be varied and thus adapted to the thermal requirements and the size of the catalyst particles. With centripetal flow through the The reaction mixture flows over several nozzles (06) into the outer concentric distributor annulus (07) of the reactor, which is formed by the pressure vessel wall (08) and a circumferential shirt (09) connected to it.

This shirt separates the annulus from the catalyst bed. The reaction gas mixture is distributed and reaches the catalyst bed through holes. It flows through it in the radial direction up to the inner collector tube (10), which also has passage openings. From here, the production gas mixture is led out of the reactor via outlet connection (11). If a reverse flow direction is to be realized, this can be done with the same construction.

A design is also possible in which part of the heated cooling medium (for example in the form of water vapor) is used to heat up cold reactor inlet gas. For this purpose, one or more of the concentric windings (12) can be decoupled from the cooling circuit - wound cooling pipes (03) - and brought together to form a heating circuit through which recirculated, heated cooling medium flows (13). This is possible with a centripetal, but also with a corresponding arrangement of the decoupled windings with a centrifugal flow direction.

The reactor with a heat exchanger with wound tubes has significantly better thermal properties than the conventional design with straight tube bundles with longitudinal flow. This has the advantage that the reactor can be built significantly more compact with the same production output and dimensions, weight and investment costs are significantly reduced. Another advantage is that with this type of construction an increase in the production capacity is possible by enlarging the reactor, while with the sizes realized today with straight tube bundles the limit of economically reasonable has been reached. Finally, the structural design with a coiled tube bundle is more advantageous with regard to the thermal stresses which are caused by temperature differences between the reaction gas mixture and the cooling medium, that is to say between the reactor pressure vessel wall and the cooling tube wall. Due to the elastic spring behavior of the coiled tubes, large temperature differences can be tolerated without causing mechanical stresses, which is important for normal operation, but above all for the reliable control of malfunctions is advantageous. The reactor and process according to the invention are therefore also associated with a not inconsiderable increase in operational safety compared to the prior art.

A further possibility for dissipating the heat of reaction occurring in the case of exothermic reactions in the reactor and process according to the invention is their direct use for heating the cold reaction gas mixture by the process of the so-called fresh gas feed. The number and the position of the fresh gas outlet points in the catalyst bed can vary depending on the amount of heat generated and to be dissipated in the catalyst bed at the given location. This method reduces the amount of heat to be removed from the reactor by active cooling or via a separate heat exchanger. The technical facilities required for active cooling or a separate heat exchanger can therefore be dimensioned smaller and the costs incurred in their manufacture and assembly are lower.

A combination of the cooling methods described above, which is used in a reactor with radial flow of the reaction gas mixture through the catalyst bed, can also be used in exothermic reactions. Heat that is not consumed by introducing cold reactor inlet gas is dissipated by a cooling medium flowing through the catalyst bed in coiled tubes.

The process according to the invention for carrying out a heterogeneous catalytic gas phase reaction can be carried out either in a cycle gas mode or with a single passage of the reaction gas mixture through the catalyst bed. In the cycle gas mode, the reaction gas mixture is sent in a cycle through the catalyst-filled reactor, formed products as well as by-products and substances that would otherwise accumulate in the cycle gas are removed from the cycle by suitable measures and the appropriate amounts of starting materials and, if appropriate, reaction modifiers are added to the gas mixture admixed again through the reactor. In the embodiment with a single gas passage through the reactor, the entire gas mixture is worked up after the reactor.

It is also possible with the reactor and process according to the invention to carry out endothermic reactions. A heating medium then flows in the tubes of the heat exchanger; the advantageous thermal properties of the reactor according to the invention described above apply to a heat flow in the opposite direction, as is required in endothermic reactions, as well as for exothermic reactions.

Claims (20)

Reactor for carrying out heterogeneous catalytic gas phase reactions, characterized in that a reaction gas mixture flows radially to the longitudinal axis of the reactor and preferably centripetal through the catalyst bed, and that the heat of reaction generated by means of a tube bundle heat exchanger built into the catalyst bed, consisting of one or more wound tubes, through which a cooling medium flows, is discharged. A method according to claim 1, characterized in that, in the case of a centripetal flow direction, the reaction gases introduced into the reactor or, in the case of a centrifugal flow direction, the reaction gases emerging from the catalyst bed enter a distributor ring space arranged around the catalyst bed, which is formed by the pressure vessel wall and a circumferential shirt connected to it is provided with through openings. A method according to claim 1, characterized in that, in the case of a centripetal flow direction, the reaction gases emerging from the catalyst bed or, in the case of a centrifugal flow direction, the reaction gases introduced into the reactor enter an internal collecting tube which is arranged concentrically to the longitudinal axis and is provided with passage openings and is connected via connecting lines from outside the Reactor can be started. A method according to claim 1, characterized in that a reaction gas mixture according to one of the embodiments described in claims 2 or 3 flows through a loose catalyst bed radially to the longitudinal axis of the reactor and preferably centripetal, and that the heat of reaction generated by introducing cold reaction gases into the reactor according to the principle the cold gas quenching is consumed. Process according to claims 1 to 4, characterized in that the heat of reaction released by a combination of the cooling methods mentioned in claims 1 and 4 - A tube bundle heat exchanger installed in the catalyst bed with one or more wound tubes in which a cooling medium flows and the principle of cold gas quenching is removed. Method according to claims 1 to 3 and 5, characterized in that bundles of several concentrically wound cooling tubes are arranged in layers, the screw direction of adjacent bundles can change and the pitch of the tube winding and / or the vertical and horizontal spacing of adjacent tube windings can vary and the thermal requirements can be adapted. Process according to claims 1 to 3, 5 and 6, wherein the gradient of the tube windings and / or the distance between adjacent tube windings in the vicinity of the outer boundary of the catalyst bed is smaller in the case of centripetal flow of the reaction gas mixture. Process according to claims 1 to 3, 5 and 6, wherein the gradient of the tube windings and / or the distance between adjacent tube windings near the inner boundary of the catalyst bed is smaller in the case of centrifugal flow of the reaction gas mixture. Process according to claims 1 to 3, 5 to 8, characterized in that the cold reaction gas mixture is heated by recirculated heated cooling medium which flows through one or more external tube windings in the case of centripetal flow direction and one or more internal tube windings in the case of centrifugal flow direction and which are decoupled from the cooling tube bundle. Process according to Claims 1 to 9, characterized in that the reaction gas mixture used for the synthesis is passed through the reactor in a circuit, with products of value and by-products of the reaction formed after passage through the reactor and any substances which otherwise accumulate in the cycle gas being taken by suitable measures are removed from the circuit and the feedstock and, if appropriate, reaction modifiers are then added to the gas mixture again before the passage through the reactor again by suitable measures. Process according to claims 1 to 10 for the production of ethylene oxide from ethylene and oxygen. Process according to claims 1 to 10 for the production of acrylic acid from acrolein and oxygen. Process according to claims 1 to 10 for the production of methacrylic acid from methacrolein and oxygen. Process according to claims 1 to 10 for the production of styrene by dehydration of methylphenylcarbinol. Process according to claims 1 to 10 for the production of methanol from synthesis gas. Process according to claims 1 to 10 for the methanation of synthesis gas. Process according to claims 1 to 10 for low-temperature conversion. Process according to claims 1 to 10 for the reaction of water with olefins to alcohols. Process according to claims 1 to 10 for the oxychlorination of olefins or aromatics. Process according to claims 1 to 10 for the production of methylamines from methanol and ammonia.

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